Migrating data from a large extent pool to a small extent pool

ABSTRACT

A computer-implemented method according to one embodiment includes identifying a request to migrate data associated with a volume from a source storage pool having a first rank extent size to a destination storage pool having a second rank extent size smaller than the first rank extent size, creating a correspondence between logical volume extents of the volume and physical offset locations within rank extents of the source storage pool, and migrating data from one or more ranks of the source storage pool to one or more ranks of the destination storage pool, utilizing the correspondence between the logical volume extents of the volume and the physical offset locations within the rank extents of the source storage pool.

BACKGROUND

The present invention relates to data migration, and more specifically,this invention relates to migrating data from a first extent pool to asecond extent pool with a smaller granularity than the first extentpool.

Extent pools in storage products may have different extent sizes. Forexample, a 1 GB extent size may be implemented for a large extent pooland a 16 MB extent size may be implemented for a small extent pool. Someworkloads may perform better in a small extent pool when compared to alarge extent pool. However, current methods to transfer data areexpensive, and involve using a host to copy the data or implementing aflash copy. There is therefore a need to efficiently migrate volumesfrom large extent pools to small extent pools.

SUMMARY

A computer-implemented method according to one embodiment includesidentifying a request to migrate data associated with a volume from asource storage pool having a first rank extent size to a destinationstorage pool having a second rank extent size smaller than the firstrank extent size, creating a correspondence between logical volumeextents of the volume and physical offset locations within rank extentsof the source storage pool, and migrating data from one or more ranks ofthe source storage pool to one or more ranks of the destination storagepool, utilizing the correspondence between the logical volume extents ofthe volume and the physical offset locations within the rank extents ofthe source storage pool.

In this way, data is migrated from a source storage pool to adestination storage pool having a rank extent size smaller than the rankextent size of the source storage pool without having to use a host tocopy the data or implement a flash copy. This reduces an amount of timeand resources of one or more systems performing the data migration,which improves a performance of the one or more systems.

In one optional embodiment, creating the correspondence includesidentifying VST entries within the volume that correspond to rankextents within the source storage pool that contain the data to bemigrated, and creating a small VST for each identified VST entry.

According to another embodiment, a computer program product formigrating data from a large extent pool to a small extent pool includesa computer readable storage medium that has program instructionsembodied therewith, where the computer readable storage medium is not atransitory signal per se, and where the program instructions areexecutable by a processor to cause the processor to perform a methodincluding identifying, utilizing the processor, a request to migratedata associated with a volume from a source storage pool having a firstrank extent size to a destination storage pool having a second rankextent size smaller than the first rank extent size, creating, utilizingthe processor, a correspondence between logical volume extents of thevolume and physical offset locations within rank extents of the sourcestorage pool, and migrating, utilizing the processor, data from one ormore ranks of the source storage pool to one or more ranks of thedestination storage pool, utilizing the correspondence between thelogical volume extents of the volume and the physical offset locationswithin the rank extents of the source storage pool.

A system according to another embodiment includes a processor, and logicintegrated with the processor, executable by the processor, orintegrated with and executable by the processor, where the logic isconfigured to identify a request to migrate data associated with avolume from a source storage pool having a first rank extent size to adestination storage pool having a second rank extent size smaller thanthe first rank extent size, create a correspondence between logicalvolume extents of the volume and physical offset locations within rankextents of the source storage pool, and migrate data from one or moreranks of the source storage pool to one or more ranks of the destinationstorage pool, utilizing the correspondence between the logical volumeextents of the volume and the physical offset locations within the rankextents of the source storage pool.

A computer-implemented method according to another embodiment includesidentifying a request to migrate data associated with a volume from asource storage pool to a destination storage pool, identifying volumesegment table (VST) entries corresponding to rank extents within thesource storage pool containing the data, allocating and synchronizingsmall VSTs for the identified VST entries within the volume, allocatingone or more rank extents within the destination storage pool,transferring the data associated with the volume from the rank extentswithin the source storage pool containing the data to the one or morerank extents in the one or more ranks of the destination storage pool,updating the small VSTs to correspond to the transferred data in the oneor more rank extents in the one or more ranks of the destination storagepool, and freeing the data from the one or more rank extents within thesource storage pool.

In this way, data is migrated from a source storage pool to adestination storage pool having a rank extent size smaller than the rankextent size of the source storage pool. This may improve a performanceof applications accessing the data in the destination storage pool, asthe applications may perform better when utilizing smaller rank extents.

According to another embodiment, a computer program product formigrating data from a large extent pool to a small extent pool includesa computer readable storage medium having program instructions embodiedtherewith, where the computer readable storage medium is not atransitory signal per se, and where the program instructions areexecutable by a processor to cause the processor to perform a methodincluding identifying, utilizing the processor, a request to migratedata associated with a volume from a source storage pool to adestination storage pool, identifying, utilizing the processor, VSTentries corresponding to rank extents within the source storage poolcontaining the data, allocating and synchronizing, utilizing theprocessor, small VSTs for each of the identified VST entries within thevolume, allocating, utilizing the processor, one or more rank extentswithin the destination storage pool, transferring, utilizing theprocessor, the data associated with the volume from the rank extentswithin the source storage pool containing the data to the one or morerank extents in the one or more ranks of the destination storage pool,updating, utilizing the processor, the small VSTs to correspond to thetransferred data in the one or more rank extents in the one or moreranks of the destination storage pool, and freeing, utilizing theprocessor, the data from the one or more rank extents within the sourcestorage pool.

Other aspects and embodiments of the present invention will becomeapparent from the following detailed description, which, when taken inconjunction with the drawings, illustrate by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a network architecture, in accordance with oneembodiment.

FIG. 2 shows a representative hardware environment that may beassociated with the servers and/or clients of FIG. 1, in accordance withone embodiment.

FIG. 3 illustrates a tiered data storage system in accordance with oneembodiment.

FIG. 4 illustrates a method for migrating data from a large extent poolto a small extent pool, in accordance with one embodiment.

FIG. 5 illustrates a method for migrating data from a source storagepool to a destination storage pool having a smaller extent granularitythan the source pool, in accordance with one embodiment.

FIG. 6A illustrates an exemplary storage environment prior to a datamigration, in accordance with one embodiment.

FIG. 6B illustrates an exemplary storage environment in response to amigration request, in accordance with one embodiment.

FIG. 6C illustrates an exemplary storage environment during a datamigration from a source pool to a destination pool, in accordance withone embodiment.

FIG. 6D illustrates an exemplary storage environment after a datamigration from a source pool to a destination pool, in accordance withone embodiment.

DETAILED DESCRIPTION

The following description discloses several preferred embodiments ofsystems, methods and computer program products for migrating data from alarge extent pool to a small extent pool. Various embodiments provide amethod to create a correspondence between logical volume extents of thevolume and physical offset locations within rank extents of the sourcestorage pool, and migrate data from one or more ranks of the sourcestorage pool to one or more ranks of the destination storage pool,utilizing the correspondence.

The following description is made for the purpose of illustrating thegeneral principles of the present invention and is not meant to limitthe inventive concepts claimed herein. Further, particular featuresdescribed herein can be used in combination with other describedfeatures in each of the various possible combinations and permutations.

Unless otherwise specifically defined herein, all terms are to be giventheir broadest possible interpretation including meanings implied fromthe specification as well as meanings understood by those skilled in theart and/or as defined in dictionaries, treatises, etc.

It must also be noted that, as used in the specification and theappended claims, the singular forms “a,” “an” and “the” include pluralreferents unless otherwise specified. It will be further understood thatthe terms “includes” and/or “comprising,” when used in thisspecification, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

The following description discloses several preferred embodiments ofsystems, methods and computer program products for migrating data from alarge extent pool to a small extent pool.

In one general embodiment, a computer-implemented method includesidentifying a request to migrate data associated with a volume from asource storage pool having a first rank extent size to a destinationstorage pool having a second rank extent size smaller than the firstrank extent size, creating a correspondence between logical volumeextents of the volume and physical offset locations within rank extentsof the source storage pool, and migrating data from one or more ranks ofthe source storage pool to one or more ranks of the destination storagepool, utilizing the correspondence between the logical volume extents ofthe volume and the physical offset locations within the rank extents ofthe source storage pool.

In this way, data is migrated from a source storage pool to adestination storage pool having a rank extent size smaller than the rankextent size of the source storage pool without having to use a host tocopy the data or implement a flash copy. This reduces an amount of timeand resources of one or more systems performing the data migration,which improves a performance of the one or more systems.

In another general embodiment, creating the correspondence includesidentifying VST entries within the volume that correspond to rankextents within the source storage pool that contain the data to bemigrated, and creating a small VST for each identified VST entry.

In another general embodiment, a computer program product for migratingdata from a large extent pool to a small extent pool includes a computerreadable storage medium that has program instructions embodiedtherewith, where the computer readable storage medium is not atransitory signal per se, and where the program instructions areexecutable by a processor to cause the processor to perform a methodincluding identifying, utilizing the processor, a request to migratedata associated with a volume from a source storage pool having a firstrank extent size to a destination storage pool having a second rankextent size smaller than the first rank extent size, creating, utilizingthe processor, a correspondence between logical volume extents of thevolume and physical offset locations within rank extents of the sourcestorage pool, and migrating, utilizing the processor, data from one ormore ranks of the source storage pool to one or more ranks of thedestination storage pool, utilizing the correspondence between thelogical volume extents of the volume and the physical offset locationswithin the rank extents of the source storage pool.

In another general embodiment, a system includes a processor, and logicintegrated with the processor, executable by the processor, orintegrated with and executable by the processor, where the logic isconfigured to identify a request to migrate data associated with avolume from a source storage pool having a first rank extent size to adestination storage pool having a second rank extent size smaller thanthe first rank extent size, create a correspondence between logicalvolume extents of the volume and physical offset locations within rankextents of the source storage pool, and migrate data from one or moreranks of the source storage pool to one or more ranks of the destinationstorage pool, utilizing the correspondence between the logical volumeextents of the volume and the physical offset locations within the rankextents of the source storage pool.

In another general embodiment, a computer-implemented method includesidentifying a request to migrate data associated with a volume from asource storage pool to a destination storage pool, identifying volumesegment table (VST) entries corresponding to rank extents within thesource storage pool containing the data, allocating and synchronizingsmall VSTs for the identified VST entries within the volume, allocatingone or more rank extents within the destination storage pool,transferring the data associated with the volume from the rank extentswithin the source storage pool containing the data to the one or morerank extents in the one or more ranks of the destination storage pool,updating the small VSTs to correspond to the transferred data in the oneor more rank extents in the one or more ranks of the destination storagepool, and freeing the data from the one or more rank extents within thesource storage pool.

In this way, data is migrated from a source storage pool to adestination storage pool having a rank extent size smaller than the rankextent size of the source storage pool. This may improve a performanceof applications accessing the data in the destination storage pool, asthe applications may perform better when utilizing smaller rank extents.

In another general embodiment, a computer program product for migratingdata from a large extent pool to a small extent pool includes a computerreadable storage medium having program instructions embodied therewith,where the computer readable storage medium is not a transitory signalper se, and where the program instructions are executable by a processorto cause the processor to perform a method including identifying,utilizing the processor, a request to migrate data associated with avolume from a source storage pool to a destination storage pool,identifying, utilizing the processor, VST entries corresponding to rankextents within the source storage pool containing the data, allocatingand synchronizing, utilizing the processor, small VSTs for each of theidentified VST entries within the volume, allocating, utilizing theprocessor, one or more rank extents within the destination storage pool,transferring, utilizing the processor, the data associated with thevolume from the rank extents within the source storage pool containingthe data to the one or more rank extents in the one or more ranks of thedestination storage pool, updating, utilizing the processor, the smallVSTs to correspond to the transferred data in the one or more rankextents in the one or more ranks of the destination storage pool, andfreeing, utilizing the processor, the data from the one or more rankextents within the source storage pool.

FIG. 1 illustrates an architecture 100, in accordance with oneembodiment. As shown in FIG. 1, a plurality of remote networks 102 areprovided including a first remote network 104 and a second remotenetwork 106. A gateway 101 may be coupled between the remote networks102 and a proximate network 108. In the context of the presentarchitecture 100, the networks 104, 106 may each take any formincluding, but not limited to a LAN, a WAN such as the Internet, publicswitched telephone network (PSTN), internal telephone network, etc.

In use, the gateway 101 serves as an entrance point from the remotenetworks 102 to the proximate network 108. As such, the gateway 101 mayfunction as a router, which is capable of directing a given packet ofdata that arrives at the gateway 101, and a switch, which furnishes theactual path in and out of the gateway 101 for a given packet.

Further included is at least one data server 114 coupled to theproximate network 108, and which is accessible from the remote networks102 via the gateway 101. It should be noted that the data server(s) 114may include any type of computing device/groupware. Coupled to each dataserver 114 is a plurality of user devices 116. User devices 116 may alsobe connected directly through one of the networks 104, 106, 108. Suchuser devices 116 may include a desktop computer, lap-top computer,hand-held computer, printer or any other type of logic. It should benoted that a user device 111 may also be directly coupled to any of thenetworks, in one embodiment.

A peripheral 120 or series of peripherals 120, e.g., facsimile machines,printers, networked and/or local storage units or systems, etc., may becoupled to one or more of the networks 104, 106, 108. It should be notedthat databases and/or additional components may be utilized with, orintegrated into, any type of network element coupled to the networks104, 106, 108. In the context of the present description, a networkelement may refer to any component of a network.

According to some approaches, methods and systems described herein maybe implemented with and/or on virtual systems and/or systems whichemulate one or more other systems, such as a UNIX system which emulatesan IBM z/OS environment, a UNIX system which virtually hosts a MICROSOFTWINDOWS environment, a MICROSOFT WINDOWS system which emulates an IBMz/OS environment, etc. This virtualization and/or emulation may beenhanced through the use of VMWARE software, in some embodiments.

In more approaches, one or more networks 104, 106, 108, may represent acluster of systems commonly referred to as a “cloud.” In cloudcomputing, shared resources, such as processing power, peripherals,software, data, servers, etc., are provided to any system in the cloudin an on-demand relationship, thereby allowing access and distributionof services across many computing systems. Cloud computing typicallyinvolves an Internet connection between the systems operating in thecloud, but other techniques of connecting the systems may also be used.

FIG. 2 shows a representative hardware environment associated with auser device 116 and/or server 114 of FIG. 1, in accordance with oneembodiment. Such figure illustrates a typical hardware configuration ofa workstation having a central processing unit 210, such as amicroprocessor, and a number of other units interconnected via a systembus 212.

The workstation shown in FIG. 2 includes a Random Access Memory (RAM)214, Read Only Memory (ROM) 216, an I/O adapter 218 for connectingperipheral devices such as disk storage units 220 to the bus 212, a userinterface adapter 222 for connecting a keyboard 224, a mouse 226, aspeaker 228, a microphone 232, and/or other user interface devices suchas a touch screen and a digital camera (not shown) to the bus 212,communication adapter 234 for connecting the workstation to acommunication network 235 (e.g., a data processing network) and adisplay adapter 236 for connecting the bus 212 to a display device 238.

The workstation may have resident thereon an operating system such asthe Microsoft Windows® Operating System (OS), a MAC OS, a UNIX OS, etc.It will be appreciated that a preferred embodiment may also beimplemented on platforms and operating systems other than thosementioned. A preferred embodiment may be written using XML, C, and/orC++ language, or other programming languages, along with an objectoriented programming methodology. Object oriented programming (OOP),which has become increasingly used to develop complex applications, maybe used.

Now referring to FIG. 3, a storage system 300 is shown according to oneembodiment. Note that some of the elements shown in FIG. 3 may beimplemented as hardware and/or software, according to variousembodiments. The storage system 300 may include a storage system manager312 for communicating with a plurality of media on at least one higherstorage tier 302 and at least one lower storage tier 306. The higherstorage tier(s) 302 preferably may include one or more random accessand/or direct access media 304, such as hard disks in hard disk drives(HDDs), nonvolatile memory (NVM), solid state memory in solid statedrives (SSDs), flash memory, SSD arrays, flash memory arrays, etc.,and/or others noted herein or known in the art. The lower storagetier(s) 306 may preferably include one or more lower performing storagemedia 308, including sequential access media such as magnetic tape intape drives and/or optical media, slower accessing HDDs, sloweraccessing SSDs, etc., and/or others noted herein or known in the art.One or more additional storage tiers 316 may include any combination ofstorage memory media as desired by a designer of the system 300. Also,any of the higher storage tiers 302 and/or the lower storage tiers 306may include some combination of storage devices and/or storage media.

The storage system manager 312 may communicate with the storage media304, 308 on the higher storage tier(s) 302 and lower storage tier(s) 306through a network 310, such as a storage area network (SAN), as shown inFIG. 3, or some other suitable network type. The storage system manager312 may also communicate with one or more host systems (not shown)through a host interface 314, which may or may not be a part of thestorage system manager 312. The storage system manager 312 and/or anyother component of the storage system 300 may be implemented in hardwareand/or software, and may make use of a processor (not shown) forexecuting commands of a type known in the art, such as a centralprocessing unit (CPU), a field programmable gate array (FPGA), anapplication specific integrated circuit (ASIC), etc. Of course, anyarrangement of a storage system may be used, as will be apparent tothose of skill in the art upon reading the present description.

In more embodiments, the storage system 300 may include any number ofdata storage tiers, and may include the same or different storage memorymedia within each storage tier. For example, each data storage tier mayinclude the same type of storage memory media, such as HDDs, SSDs,sequential access media (tape in tape drives, optical disk in opticaldisk drives, etc.), direct access media (CD-ROM, DVD-ROM, etc.), or anycombination of media storage types. In one such configuration, a higherstorage tier 302, may include a majority of SSD storage media forstoring data in a higher performing storage environment, and remainingstorage tiers, including lower storage tier 306 and additional storagetiers 316 may include any combination of SSDs, HDDs, tape drives, etc.,for storing data in a lower performing storage environment. In this way,more frequently accessed data, data having a higher priority, dataneeding to be accessed more quickly, etc., may be stored to the higherstorage tier 302, while data not having one of these attributes may bestored to the additional storage tiers 316, including lower storage tier306. Of course, one of skill in the art, upon reading the presentdescriptions, may devise many other combinations of storage media typesto implement into different storage schemes, according to theembodiments presented herein.

According to some embodiments, the storage system (such as 300) mayinclude logic configured to receive a request to open a data set, logicconfigured to determine if the requested data set is stored to a lowerstorage tier 306 of a tiered data storage system 300 in multipleassociated portions, logic configured to move each associated portion ofthe requested data set to a higher storage tier 302 of the tiered datastorage system 300, and logic configured to assemble the requested dataset on the higher storage tier 302 of the tiered data storage system 300from the associated portions.

Of course, this logic may be implemented as a method on any deviceand/or system or as a computer program product, according to variousembodiments.

Now referring to FIG. 4, a flowchart of a method 400 is shown accordingto one embodiment. The method 400 may be performed in accordance withthe present invention in any of the environments depicted in FIGS. 1-3and 6A-D, among others, in various embodiments. Of course, more or lessoperations than those specifically described in FIG. 4 may be includedin method 400, as would be understood by one of skill in the art uponreading the present descriptions.

Each of the steps of the method 400 may be performed by any suitablecomponent of the operating environment. For example, in variousembodiments, the method 400 may be partially or entirely performed byone or more servers, computers, or some other device having one or moreprocessors therein. The processor, e.g., processing circuit(s), chip(s),and/or module(s) implemented in hardware and/or software, and preferablyhaving at least one hardware component may be utilized in any device toperform one or more steps of the method 400. Illustrative processorsinclude, but are not limited to, a central processing unit (CPU), anapplication specific integrated circuit (ASIC), a field programmablegate array (FPGA), etc., combinations thereof, or any other suitablecomputing device known in the art.

As shown in FIG. 4, method 400 may initiate with operation 402, where arequest is received to migrate data associated with a volume from asource storage pool having a first rank extent size to a destinationstorage pool having a second rank extent size smaller than the firstrank extent size.

Additionally, method 400 may proceed with operation 404, where acorrespondence is created between logical volume extents of the volumeand physical offset locations within rank extents of the source storagepool. In one embodiment, creating the correspondence includesidentifying VST entries within the volume that correspond to rankextents within the source storage pool that contain the data to bemigrated. For example, a VST may store a logical representation of datastored within the volume, where the VST includes a plurality of entries.A subset of these entries may be identified within the VST thatcorrespond to rank extents within the source storage pool that containthe data to be migrated.

Further, in one embodiment, creating the correspondence may includecreating a small VST for each identified VST entry. For example, foreach of the identified subset of VST entries, a small VST may becreated. In another example, each of the small VSTs may be used torepresent all of the logical volume extents within their correspondingVST entry at a higher granularity.

Further, in one embodiment, creating the correspondence may includesetting the logical volume extents within the small VSTs to point tocorresponding offset locations within the rank extents within the sourcestorage pool that contain the data to be migrated. In this way, a directcorrespondence may be established between the logical volume extents ofthe small VSTs of the volume and the offset locations within the rankextents within the source storage pool that contain the data to bemigrated.

Further still, method 400 may proceed with operation 406, where data ismigrated from one or more ranks of the source storage pool to one ormore ranks of the destination storage pool, utilizing the correspondencebetween the logical volume extents of the volume and the physical offsetlocations within the rank extents of the source storage pool. In oneembodiment, migrating the data may include selecting a plurality of rankextents within the destination storage pool to receive the migrateddata. For example, the plurality of rank extents may be selectedrandomly or according to one or more criteria (e.g., a rank numbering, adata retrieval speed of a corresponding rank, an amount of datacurrently being stored in the corresponding rank, etc.).

Also, in one embodiment, migrating the data may include transferring thedata associated with the volume from one or more offset locations withinthe rank extents of the source storage pool to the selected plurality ofrank extents within the destination storage pool.

In addition, in one embodiment, migrating the data may include freeingthe data from the one or more rank extents within the source storagepool. In another embodiment, migrating the data may include adjustingthe volume extents within the small VSTs to point to the correspondinglocations of the migrated data within the selected plurality of rankextents of the destination storage pool that store the transferred data.

In this way, data is migrated from a source storage pool to adestination storage pool having a rank extent size smaller than the rankextent size of the source storage pool without having to use a host tocopy the data or implement a flash copy. This reduces an amount of timeand resources of one or more systems performing the data migration,which improves a performance of the one or more systems.

Now referring to FIG. 5, a flowchart of a method 500 for migrating datafrom a source storage pool to a destination storage pool having asmaller extent granularity than the source pool is identified is shownaccording to one embodiment. The method 500 may be performed inaccordance with the present invention in any of the environmentsdepicted in FIGS. 1-3 and 6A-D, among others, in various embodiments. Ofcourse, more or less operations than those specifically described inFIG. 5 may be included in method 500, as would be understood by one ofskill in the art upon reading the present descriptions.

Each of the steps of the method 500 may be performed by any suitablecomponent of the operating environment. For example, in variousembodiments, the method 500 may be partially or entirely performed byone or more servers, computers, or some other device having one or moreprocessors therein. The processor, e.g., processing circuit(s), chip(s),and/or module(s) implemented in hardware and/or software, and preferablyhaving at least one hardware component may be utilized in any device toperform one or more steps of the method 500. Illustrative processorsinclude, but are not limited to, a central processing unit (CPU), anapplication specific integrated circuit (ASIC), a field programmablegate array (FPGA), etc., combinations thereof, or any other suitablecomputing device known in the art.

As shown in FIG. 5, method 500 may initiate with operation 502, where arequest to migrate data associated with a volume from a source storagepool to a destination storage pool is identified. In one embodiment, thevolume includes a storage volume that organizes and presents a logicalrepresentation of the data in a contiguous manner to one or more hosts.In another embodiment, the volume includes a plurality of volumeextents. For example, each volume extent may include a logical extent.

Additionally, in one embodiment, a storage pool includes one or moreranks. In another embodiment, each rank includes a redundant amount ofstorage (e.g., a RAID storage array, etc.). In yet another embodiment,data logically represented in the volume is physically stored in one ormore ranks of a storage pool. For example, in one embodiment, each rankincludes a plurality of rank extents.

Further, in one embodiment, the extents within a rank are represented asentries within a rank segment table (RST) for the rank, where each rankextent is either full or empty. For example, each rank extent representsa physical extent. In another example, each rank extent represents apredetermined amount of storage for storing data within the storagepool. In yet another example, full rank extents include a logicallocation of data within the volume (e.g., an identification of a volumeand a volume extent within that volume where the data is logicallystored and presented to one or more hosts).

Further still, in one embodiment, the source storage pool includes oneor more ranks that initially store all data logically represented in thevolume. In another embodiment, the data logically represented in thevolume is physically stored in multiple ranks within the source storagepool.

Also, in one embodiment, the source storage pool has a first rank extentsize, and the destination storage pool has a second rank extent sizesmaller than the first rank extent size. For example, the source storagepool may have rank extents with a size of 1 GB, and the second extentpool may have rank extents with a size of 16 MB.

In addition, method 500 may proceed with operation 504, where VSTentries corresponding to rank extents within the source storage poolcontaining the data are identified. In one embodiment, the volumeextents are grouped within entries of a volume segment table (VST),where each volume extent is either full or empty. For example, in oneembodiment, full volume extents include a physical location of datawithin the source storage pool (e.g., an identification of a rank and arank extent within that rank where the data associated with the volumeextent is stored). In another example, each entry of a VST includes apredetermined number of consecutive volume extents. In yet anotherexample, the VST maps the logical extents of the volume (e.g., thevolume extents) to physical extents of one or more ranks within astorage pool (e.g., rank extents).

Furthermore, method 500 may proceed with operation 506, where small VSTsare allocated and synchronized for the identified VST entries within thevolume. In one embodiment, small VSTs represent and manage the entriesof the VST with a higher granularity. For example, each entry of the VSTincludes a grouped plurality of volume extents identified only by a VSTentry number. In another example, a single VST entry may be representedby a small VST having entries corresponding to each of the plurality ofvolume extents grouped within the single VST entry.

Further still, in one embodiment, synchronizing the small VSTs mayinclude setting the logical volume extents within each of the small VSTsto point to corresponding offset locations within the rank extentswithin the source storage pool that contain the data to be migrated. Forexample, the offset locations of a rank extent represent a logical rankextent number within a logical representation of the rank extent (e.g.,that indicates a predetermined portion of the rank extent). In anotherexample, a rank extent of size 1 GB may have a plurality of offsetlocations that each represent 16 MB portions of the rank extent (suchthat a single 1 GB rank extent would have sixty-four 16 MB offsetlocations, etc.). This allows for a representation of a rank extent at ahigher granularity.

In another embodiment, synchronizing the small VSTs may includepopulating the small VSTs so that they include volume extents thatcontain offset locations within a corresponding rank extent within thesource storage pool. In this way, the entries within the small VSTs arepopulated such that an organization (e.g., order, numbering, etc.) ofentries within the small VSTs match an organization of offset locationswithin allocated rank extents corresponding to the small VSTs.

Also, method 500 may proceed with operation 508, where one or more rankextents are allocated within the destination storage pool. In oneembodiment, a number of rank extents allocated within the destinationstorage pool corresponds to a size of the volume. For example, a totalsize of data logically represented by the volume is allocated within thedestination storage pool. In another example, the total size of the datalogically represented by the volume is divided by a rank extent size ofthe destination storage pool to determine a number of rank extents thatare allocated within the destination storage pool. In anotherembodiment, the one or more rank extents are allocated randomly,serially, non-serially, etc.

Additionally, in one embodiment, the one or more rank extents may beallocated according to one or more criteria. For example, the one ormore rank extents may be allocated based on a data retrieval speed ofthe ranks within the destination storage pool. For instance, each of theranks may be listed in order of decreasing data retrieval speed, andextents may be allocated in ranks according to the list (e.g., startingwith ranks having a fastest data retrieval speed, etc.).

In another example, the one or more rank extents may be allocated basedon an amount of data of data currently being stored in each of the rankswithin the destination storage pool. For instance, the ranks may belisted in order of a decreasing amount of currently available storagespace, and extents may be allocated in ranks according to the list(e.g., starting with ranks having most currently available extents,etc.).

In addition, method 500 may proceed with operation 510, where the dataassociated with the volume is transferred from the rank extents withinthe source storage pool containing the data to the one or more rankextents in the one or more ranks of the destination storage pool. In oneembodiment, transferring the data includes determining a location of alloffset locations within the rank extents of one or more RSTs of thesource storage pool that correspond to all previously allocated volumeextents within the volume. In another embodiment, transferring the dataincludes determining all allocated rank extents within the destinationstorage pool.

Further still, in one embodiment, transferring the data includestransferring the data stored in the offset locations of the rank extentswithin the source storage pool to the corresponding allocated rankextents within the destination storage pool. For example, data stored inan offset location of a rank extent of a rank of the source storage poolis transferred to an allocated rank extent of the destination storagepool.

Also, method 500 may proceed with operation 512, where the small VSTsare updated to correspond to the transferred data in the one or morerank extents in the one or more ranks of the destination storage pool.For example, volume extents are adjusted within small VSTs of the volumeto point to the corresponding locations within the allocated one or morerank extents of the destination storage pool that store the transferreddata.

Additionally, method 500 may proceed with operation 514, where the datais freed from the one or more rank extents within the source storagepool. In one embodiment, freeing the data may include deleting the datafrom the one or more rank extents within the source storage pool.

For example, a volume extent within a small VST originally identifies anoffset location within a logical representation of a rank extent in thesource storage pool where the data logically represented by that volumeextent is physically stored. In another example, the volume extent isupdated within the small VST to identify the allocated rank extent ofthe destination storage pool where the data associated with the volumeextent was migrated. In yet another example, the stored data is migratedfrom the offset location within the logical representation of the rankextent in the source storage pool to an allocated rank extent of thedestination storage pool.

In this way, volume extents within the small VSTs of the volume matchthe allocated rank extents in the destination storage pool where dataassociated with those volume extents is stored. For example, a locationof a volume extent within a small VST of the volume matches an allocatedrank extent where data for the entry is stored. This simplifies andexpedites data recall within the destination storage pool.

In one embodiment, one or more applications are provided access to thedata at the destination storage pool. For example, one or moreapplications running on one or more hosts may send one or more datarequests to the volume, and the volume may direct the data requests tothe destination storage pool to implement data access. In anotherembodiment, the one or more applications may run more efficiently whenaccessing data using the smaller rank extent granularity within thedestination storage pool compared to the larger rank extent granularitywithin the source storage pool.

In this way, data is migrated from a source storage pool to adestination storage pool having a rank extent size smaller than the rankextent size of the source storage pool. This may improve a performanceof applications accessing the data in the destination storage pool, asthe applications may perform better when utilizing smaller rank extents.

FIG. 6A illustrates an exemplary storage environment 600 prior to a datamigration, according to one embodiment. As shown, a VST 602 stores alogical representation of data stored within a volume 0 604.Additionally, RSTs store a physical representation of the data as storedwithin a rank 3 624 of a source pool 0 626. Also, the rank granularityof source pool 0 626 is 1 GB. For example, each rank extent withinsource pool 0 626 has a size of 1 GB.

Further, full volume extent VST entries 632 and 634 refer to locations630A-B within the RSTs where data is physically stored within sourcepool 0 626. For example, rank extent 0 630A of rank 3 624 of source pool0 626 includes a link to VST entry 0 632 within volume 0 604.Conversely, VST entry 0 632 within volume 0 604 includes a link to rankextent 0 630A of rank 3 624 of source pool 0 626.

Likewise, rank extent 64 630B of rank 3 624 of source pool 0 626includes a link to VST entry 2 634 within volume 0 604. Conversely, VSTentry 2 634 within volume 0 604 includes a link to rank extent 64 630Bof rank 3 624 of source pool 0 626.

In this way, a connection between a logical representation of data involume 0 604 is mapped to a physical representation of data in sourcepool 0 626.

FIG. 6B illustrates an exemplary storage environment 600 in response toa migration request, according to one embodiment. The rank granularityof a destination pool 1 622 is 16 MB. For example, each rank extentwithin the destination pool 1 622 has a size of 16 MB. Since the rankgranularity of the source pool 0 626 is 1 GB, the destination pool 1 622has a smaller rank granularity than the source pool 0 626.

In one embodiment, a request is received to migrate data from sourcepool 0 626 to destination pool 1 622. In another embodiment, in responseto the migration request, small VSTs 614 and 616 are created withinvolume 0 604. For example, small VSTs 614 and 616 are used to representthe volume extents within VST entry 0 632 and VST entry 2 634,respectively.

Additionally, in one embodiment, extents 618A-E and 619A-E within smallVSTs 614 and 616 are updated to include links to corresponding offsetlocations 640A-E and 641A-E of allocated rank extents 630A and 630B.Logical representations 636 and 638 of offset locations 640A-E and641A-E of allocated rank extents 630A and 630B are shown. For example,volume extent 0 618A of small VST 614 may be updated to point to offsetlocation 0 640A of logical representation 636 of allocated rank extent 0630A, volume extent 1 618B of small VST 614 may be updated to point tooffset location 1 640B of logical representation 636 of allocated rankextent 0 630A, etc.

In this way, a correlation may be made between the extents 618A-E and619A-E within small VSTs 614 and 616 and offset locations 640A-E and641A-E of allocated rank extents 630A and 630B.

FIG. 6C illustrates an exemplary storage environment 600 during a datamigration from a source pool 0 626 to a destination pool 1 622,according to one embodiment. As shown, rank extents 620A-E and 621A-Eare allocated within destination rank 0 610 and destination rank 1 612in the destination pool 1 622. It should be noted that although theallocated rank extents 620A-E and 621A-E are serial, allocation may notbe performed serially.

Additionally, although destination rank 0 610 and destination rank 1 612are shown, rank extents from other ranks within the destination pool 622may be allocated. For example, rank extents may be allocated amongstranks within the destination pool 622 randomly, according to a dataretrieval speed of the ranks, according to an amount of data currentlybeing stored in the ranks, etc.

Further, as shown, data physically stored in offset location 0 640A ofrank 0 630A of rank 3 624 has been transferred to rank extent 1 621B ofdestination rank 1 612, and volume extent 0 618A of the small VST 614 isupdated to point to rank extent 1 621B. Conversely, rank extent 1 621Bof destination rank 1 612 of source pool 1 622 is updated to include alink to volume extent 0 618A within volume 0 604. This results in staledata at offset location 0 640A.

Similarly, as shown, data physically stored in offset location 2 640Chas been transferred to rank extent 1 620B of destination rank 0 610,and volume extent 2 618C of the small VST 614 is updated to point torank extent 1 620B. Conversely, rank extent 1 620B of destination rank 0610 of source pool 1 622 is updated to include a link to volume extent 2618C within volume 0 604. This results in stale data at offset location2 640C.

Further, as shown, data physically stored in offset location 65 641B hasbeen transferred to rank extent 0 621A of destination rank 1 612, andvolume extent 129 619B of the small VST 616 is updated to point to rankextent 0 621A. Conversely, rank extent 0 621A of destination rank 1 612of source pool 1 622 is updated to include a link to volume extent 129619B within volume 0 604. This results in stale data at offset location65 641B.

Also, as shown, data physically stored in offset location 127 641E hasbeen transferred to rank extent 2 620C of destination rank 0 610, andvolume extent 191 619E of the small VST 616 is updated to point to rankextent 2 620C. Conversely, rank extent 2 620C of destination rank 0 610of source pool 1 622 is updated to include a link to volume extent 191619E within volume 0 604. This results in stale data at offset location127 641E.

FIG. 6D illustrates an exemplary storage environment 600 after a datamigration from a source pool 0 626 to a destination pool 1 622,according to one embodiment. As shown, all data physically stored inrank extents 0 630A and 64 630B of source pool 0 626 is transferred toallocated rank extents within rank 0 610 and rank 1 612 of destinationpool 1 622. Additionally, corresponding volume extents in small VSTs 614and 616 are updated to point to the corresponding rank extents that nowstore the migrated data.

This results in stale data in all offset locations of rank extents 0630A and 64 630B, and data in such rank extents 0 630A and 64 630B isfreed as a result.

In this way, data may be migrated from the source pool 0 626 to thedestination pool 1 622, and the volume 0 604 may be updated to reflectthe migration.

Migration of Volumes from a Large Extent Pool to a Small Extent Pool

Extent pools in storage products could have different extent size. Forexample, DS8000 may use a 1 GB extent size for a large pool and a 16 MBfor a small pool. Some workloads perform better using small pools, whileothers perform better in large extent pools. There is therefore a needto migrate volumes across different sized extent pools as the workloadchanges.

Extent migration is a building block of volume migration. At thebeginning of extent migration, rank extents are allocated in the targetextent pool as the migration target of the source extent. If the extentsize is the source and target extent pool are same, the migrationprocess includes allocating one extent in the target pool, migrating thedata from a source to a target, holding backend I/O as migration isongoing (or performing a mirror write), and switching I/O to the targetonce migration is complete.

A method is disclosed to migrate data from a large extent pool to asmall extent pool in an efficient way, where migration is able to beresumed when it is interrupted (planned or unplanned).

In one embodiment, a Rank Segment Table (RST) is a mapping table for arank (array) describing the state of rank extents and the logical volumeextents that the physical rank extents belong to. For example, say rankrO is 1 T, and the extent size is 1G, then this RST table would have1024 entries, with each entry representing one physical extent. If rankextent 0 is allocated to logical extent 1 of volume 0x1010, then theentry in RST will have information indicating that the physical extentstate is allocated, and it is allocated to logical extent 1 of volume0x1010.

Additionally, in one embodiment, a Volume Segment Table (VST) is amapping table for volumes that maps the logical extent of the volume tothe physical extent of the rank, so that we can easily identify thephysical location when host I/O arrives which is on a volume LBAboundary.

Further, in one embodiment, a Small Volume Segment Table (smVST) is alsoa mapping table for a volume that contains 64 entries with each entrymap a small logical extent (16 MB) to the physical extent of the rank,so that we can easily identify a physical location when a host I/Oarrives which is on a volume LBA boundary.

Further still, in one embodiment, only an RST is persistent in disks,and a VST is built and loaded into memory on demand based on the RST.

Also, in one embodiment, the following steps provide for extentmigration from a large extent pool to a small extent pool:

-   -   1. Pre-check to make sure the migration request can be        fulfilled.    -   2. Quiesce Thin Provisioning write if it is a Thin Provisioning        volume.    -   3. Update volume extent pool ID to targeting extent pool, turn        on volume flag to indicate space allocation in progress and        harden the volume structure.    -   4. Allocate targeting RST objects for customer data.    -   5. As for Thin Provisioning volume, if the source large extent        is already allocated, allocate 64 small extents in the targeting        extent pool and marked them as DSR_TGT. If the source extent is        not allocated, there is no need to allocate small extents in the        target pool.    -   6. Allocate targeting RST objects for meta data.    -   7. Resume Thin Provisioning write if it is a Thin Provisioning        volume.    -   8. Harden changed RST objects by using an RST changed bitmap.    -   9. Quiesce Thin Provisioning write if it is a Thin Provisioning        volume.    -   10. Allocate smVST if all entries are in an unallocated state.    -   11. Send mail to peer LPAR to sync smVST (This step is valid        only for ESE volumes).    -   12. Resume Thin Provisioning write if it is a Thin Provisioning        volume.    -   13. Turn off a volume allocation flag, turn on a volume        migration flag and harden a volume structure.    -   14. Proceed to migrate volume extents.    -   15. Find a small extent in DSR_TGT state in the target pool and        pair it with the source small extent chunk to start small extent        migration; hold I/O in 16 MB unit.    -   16. Switch to a metadata update phase once the 16 MB customer        data from the source extent is all in the target.    -   17. Change and harden the state of target RST to TGT_CMPLT        state.    -   18. Allocate a smVST if smVST does not yet exist.    -   19. Initialize smVST entries to point to the related small        chunks in a source large extent.    -   20. Update smVST to point to the new target small extent on all        available clusters.    -   21. Release all held stage & destage I/O.    -   22. CACHE SCAN to have CACHE destage all modified tracks in the        extent.    -   23. Free the source extent if it is the last small chunk to        migrate to the target, otherwise, go back to step 15 to handle        the next small chunk.    -   24. Change and harden the state of target RST to an ALLOCATED        state.

Method to Allocate Space and the Timing to Allocate/Update smVST MappingTable

In one embodiment, space is pre-allocated in the targeting extent poolbefore migration starts. 1 GB large extent equals 64 16 MB smallextents, so if the source large extent is allocated, there will be 64small extents pre-allocated in the target pool. If the source largeextent is not allocated yet, there is no need to provision space in thetarget pool either.

A smVST entry should be allocated for the 64 small extent chunks if thesource is not allocated yet; that way, once I/O comes in writing to anyof the 64 small extent chunks, we can allocate a small extent from thetarget small extent pool and update the smVST entry to point to thetarget small extent. The smVST update needs to be synced between storagenodes.

If the source large extent is in an allocated state, we can either stillallocate the smVST beforehand, and update its entries to point to therelated small chunks in the large extent, or delay the allocation ofsmVST to when the first 16 MB small chunk of the large extent has beenmigrated to the target small extent.

Method to Hold Backend I/O

In One Embodiment, the Backend I/O is Held in a 16 MB Small Extent Unit.During the migration of the 16 MB small chunk from the large extent tothe related small extent in the target pool, backend I/O is written toboth the source and target as migration is being performed from thesource to target. Switching to a metadata update phase is performed oncethe customer data from the source small chunk is mostly in the target.Changing the target RST state to a TGT_CMPLT state indicates the copy iscomplete, and updating the related smVST entry is performed to point tothe target extent so that I/O can be directed to the target afterwards.

Method to Handle Migration Interrupt Due to Planned/Unplanned Reboot

When migration is interrupted by a planned/unplanned reboot, part of the1 GB data is in the source pool and the others are in the target pool.How do we choose which copy to use? How do we build the VST to map tothe correct rank segments? In one embodiment, small always triumphs: Theprocess may include scanning ranks and building a VST for each RSTthat's either in a TGT_CMPLT or ALLOCATED state. An RST from small ranksalways overwrites an RST from large ranks. This is because the I/O hasbeen switched to target a small extent if the small extent is already ina TGT_CMPLT or ALLOCATED state. Therefore, we need to overwrite therelated smVST to point to the target small extent.

The present invention may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein includes anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which includes one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

Moreover, a system according to various embodiments may include aprocessor and logic integrated with and/or executable by the processor,the logic being configured to perform one or more of the process stepsrecited herein. By integrated with, what is meant is that the processorhas logic embedded therewith as hardware logic, such as an applicationspecific integrated circuit (ASIC), a FPGA, etc. By executable by theprocessor, what is meant is that the logic is hardware logic; softwarelogic such as firmware, part of an operating system, part of anapplication program; etc., or some combination of hardware and softwarelogic that is accessible by the processor and configured to cause theprocessor to perform some functionality upon execution by the processor.Software logic may be stored on local and/or remote memory of any memorytype, as known in the art. Any processor known in the art may be used,such as a software processor module and/or a hardware processor such asan ASIC, a FPGA, a central processing unit (CPU), an integrated circuit(IC), a graphics processing unit (GPU), etc.

It will be clear that the various features of the foregoing systemsand/or methodologies may be combined in any way, creating a plurality ofcombinations from the descriptions presented above.

It will be further appreciated that embodiments of the present inventionmay be provided in the form of a service deployed on behalf of acustomer to offer service on demand.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Thus, the breadth and scope of a preferred embodiment shouldnot be limited by any of the above-described exemplary embodiments, butshould be defined only in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A computer-implemented method, comprising:identifying a request to migrate data associated with a volume from asource storage pool having a first rank extent size to a destinationstorage pool having a second rank extent size smaller than the firstrank extent size; creating a correspondence between logical volumeextents of the volume and physical offset locations within rank extentsof the source storage pool; and migrating data from one or more ranks ofthe source storage pool to one or more ranks of the destination storagepool, utilizing the correspondence between the logical volume extents ofthe volume and the physical offset locations within the rank extents ofthe source storage pool.
 2. The computer-implemented method of claim 1,wherein migrating the data includes transferring the data associatedwith the volume from one or more offset locations within the rankextents of the source storage pool to the rank extents within thedestination storage pool.
 3. The computer-implemented method of claim 1,wherein migrating the data includes freeing the data from the rankextents within the source storage pool.
 4. The computer-implementedmethod of claim 1, wherein migrating the data includes adjusting thelogical volume extents to point to a corresponding location of migrateddata within rank extents of the destination storage pool.
 5. Thecomputer-implemented method of claim 1, wherein creating thecorrespondence includes identifying VST entries within the volume thatcorrespond to rank extents within the source storage pool that containthe data to be migrated.
 6. The computer-implemented method of claim 5,wherein creating the correspondence includes creating a small VST foreach identified VST entry.
 7. The computer-implemented method of claim6, wherein creating the correspondence includes setting the logicalvolume extents within each small VST to point to corresponding offsetlocations within the rank extents within the source storage pool thatcontain the data to be migrated.
 8. The computer-implemented method ofclaim 1, wherein migrating the data includes selecting a plurality ofrank extents within the destination storage pool to receive migrateddata.
 9. The computer-implemented method of claim 8, wherein theplurality of rank extents are selected randomly.
 10. Thecomputer-implemented method of claim 8, wherein the plurality of rankextents are selected according to a data retrieval speed of acorresponding rank.
 11. The computer-implemented method of claim 8,wherein the plurality of rank extents are selected according to anamount of data currently being stored in a corresponding rank.
 12. Acomputer program product for migrating data from a large extent pool toa small extent pool, the computer program product comprising a computerreadable storage medium having program instructions embodied therewith,wherein the computer readable storage medium is not a transitory signalper se, the program instructions executable by a processor to cause theprocessor to perform a method comprising: identifying, utilizing theprocessor, a request to migrate data associated with a volume from asource storage pool having a first rank extent size to a destinationstorage pool having a second rank extent size smaller than the firstrank extent size; creating, utilizing the processor, a correspondencebetween logical volume extents of the volume and physical offsetlocations within rank extents of the source storage pool; and migrating,utilizing the processor, data from one or more ranks of the sourcestorage pool to one or more ranks of the destination storage pool,utilizing the correspondence between the logical volume extents of thevolume and the physical offset locations within the rank extents of thesource storage pool.
 13. The computer program product of claim 12,wherein migrating the data includes transferring the data associatedwith the volume from one or more offset locations within the rankextents of the source storage pool to the rank extents within thedestination storage pool, utilizing the processor.
 14. The computerprogram product of claim 12, wherein migrating the data includes freeingthe data from the rank extents within the source storage pool, utilizingthe processor.
 15. The computer program product of claim 12, whereinmigrating the data includes adjusting the logical volume extents topoint to a corresponding location of migrated data within rank extentsof the destination storage pool, utilizing the processor.
 16. Thecomputer program product of claim 12, wherein creating thecorrespondence includes identifying VST entries within the volume thatcorrespond to rank extents within the source storage pool that containthe data to be migrated, utilizing the processor.
 17. The computerprogram product of claim 16, wherein creating the correspondenceincludes creating a small VST for each identified VST entry, utilizingthe processor.
 18. The computer program product of claim 17, whereincreating the correspondence includes setting the logical volume extentswithin each small VST to point to corresponding offset locations withinthe rank extents within the source storage pool that contain the data tobe migrated, utilizing the processor.
 19. The computer program productof claim 12, wherein migrating the data includes selecting a pluralityof rank extents within the destination storage pool to receive migrateddata, utilizing the processor.
 20. A system, comprising: a processor;and logic integrated with the processor, executable by the processor, orintegrated with and executable by the processor, the logic beingconfigured to: identify a request to migrate data associated with avolume from a source storage pool having a first rank extent size to adestination storage pool having a second rank extent size smaller thanthe first rank extent size; create a correspondence between logicalvolume extents of the volume and physical offset locations within rankextents of the source storage pool; and migrate data from one or moreranks of the source storage pool to one or more ranks of the destinationstorage pool, utilizing the correspondence between the logical volumeextents of the volume and the physical offset locations within the rankextents of the source storage pool.